Silicon (Si) is conventionally used as a material for power semiconductor devices controlling high voltage and large current. The power semiconductor devices fall into several types, such as bipolar transistors, insulated gate bipolar transistors (IGBTs), and metal-oxide semiconductor field-effect transistors (MOSFET), and are selectively used according to the intended use.
For example, bipolar transistors and IGBTs cannot be switched at high speed although higher current density enables larger current as compared to MOSFETs. For example, the use of bipolar transistors is limited up to a switching frequency of several kHz and the use of IGBTs is limited up to tens of kHz. On the other hand, power MOSFETs can perform high-speed switching operation, although lower current density makes it difficult to handle larger current as compared to bipolar transistors and IGBTs.
Nonetheless, since a power semiconductor device supporting both large current and high-speed performance is strongly demanded in the market, efforts are made to improve IGBTs and power MOSFETs, which have been substantially developed closely to the material limits the performance of power devices substantially reaches the theoretical limit decided by materials. Semiconductor materials replacing silicon are studied in terms of power semiconductor devices and silicon carbide (SiC) is attracting attention as a semiconductor material that can be used for producing (fabricating) a next generation power semiconductor device having excellent low ON-voltage, high-speed characteristics, and high-temperature characteristics (see Non-Patent Literature 1).
Chemically, silicon carbide is a very stable semiconductor material with a wide band gap of 3 eV and can be used very stably as a semiconductor even at high temperatures. Silicon carbide has a critical electric field that is at least 10-fold that of silicon and therefore, is expected to be used as a semiconductor material capable of making ON-resistance sufficiently small. Such features of silicon carbide are the same as those of other wide band gap semiconductors, for example, gallium nitride (GaN). Therefore, by using wide band gap semiconductors, a higher breakdown voltage of a semiconductor device can be achieved (see, e.g., Non-Patent Literature 2).
However, in a high-voltage semiconductor device, high voltage is applied not only to an active region in which element structure is formed, but also to a breakdown voltage structure portion disposed in a peripheral portion of the active region to retain breakdown voltage and an electric field concentrates on the breakdown voltage structure portion. The breakdown voltage of a high-breakdown semiconductor device is determined by the impurity concentration, thickness, and field intensity; and breakdown tolerance, which is determined by semiconductor specific features, in this way is equal across the active region and the breakdown voltage structure portion. Therefore, when an electric field concentrates on the breakdown voltage structure portion, an electric load exceeding the breakdown tolerance may be applied to the breakdown voltage structure portion and may cause destruction.
A semiconductor device having a breakdown voltage structure portion provided with a termination structure such as a junction termination (junction termination extension (JTE)) structure and a field limiting ring (FLR) structure is known as a high-voltage semiconductor device that improves the breakdown voltage of the entire device by alleviating or dispersing the electric field of the breakdown voltage structure portion. In a known semiconductor device, a floating metal electrode in contact with the FLR is disposed as a field plate (FP) to release electric charge generated in the breakdown voltage structure portion to improve reliability (see, e.g., Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-50147    Non-Patent Literature 1: K. Shenai, et al, “Optimum Semiconductors for High-Power Electronics”, IEEE Transactions on Electron Devices, Vol. 36, No. 9, September 1989, pp. 1811-1823    Non-Patent Literature 2: B. Jayant Baliga, “Silicon Carbide Power Devices”, (USA), World Scientific Publishing Co, Mar. 30, 2006, p. 61